The disclosure concerns an electrophotographic printer to print to a recording medium with toner particles of a developer mixture, which toner particles are applied with the aid of a liquid developer or dry toner mixture. In the following, liquid developer is used as an example of a developer mixture in the explanation of the exemplary embodiment, without thereby limiting the exemplary embodiment to this.
Given such printers, a charge image generated on a photoconductor is inked by means of electrophoresis with the aid of the liquid developer. The toner image that is created in such a manner is transferred onto the recording medium indirectly (via a transfer element) or directly. The liquid developer has toner particles and carrier fluid in a desired ratio. Mineral oil is advantageously used as carrier fluid. In order to provide the toner particles with an electrostatic charge, charge control substances can be added to the liquid developer. Further additives can additionally be added, for example in order to achieve the desired viscosity or a desired drying behavior of the liquid developer.
Such printers are known from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1 or DE 10 2009 060 334 A1, for example.
A print group of an electrophotographic printer essentially comprises an electrophotography station, a developer station and a transfer station. The core of the electrophotography station is a photoelectric image carrier that has on its surface a photoelectric layer (what is known as a photoconductor). For example, the photoconductor is designed as a photoconductor roller that rotates past different elements to generate a print image. The photoconductor roller is initially cleaned of all contaminants. For this, an erasure light is present that erases charges remaining on the surface of the photoconductor roller. After the erasure light, a cleaning device mechanically cleans off the photoconductor roller in order to remove toner particles that are possibly still present on the surface of the photoconductor roller, possibly dust particles and remaining carrier fluid. The photoconductor roller is subsequently charged by a charging device to a predetermined charge potential. For this, for example, the charging device has a corotron device (advantageously comprising multiple corotrons). The charge potential of the photoconductor roller is controllable by adjusting the current that is supplied to the corotron device. Arranged after the charging device is a character generator that discharges the photoconductor roller via optical radiation depending on the desired print image. A latent charge image or potential image of the print image is thereby created.
The latent charge image of the print image that is generated by the character generator is inked with charged toner particles by the developer station. For this, the developer station has a rotating developer roller that directs a layer of liquid developer onto the photoconductor roller. At the developer roller, a BIAS voltage is applied, wherein a BIAS potential develops at its surface. A developer gap exists between the rollers, in which developer gap an electrical field is generated due to the developer voltage (formed by the difference between the BIAS potential at the developer roller and the discharge potential at the photoconductor roller) applied at the developer gap, due to which electrical field the charged toner particles electrophoretically migrate from the developer roller onto the photoconductor roller at the image points on the photoconductor roller. No toner passes onto the photoconductor roller in the non-image points because the direction of the electrical field (that results from the BIAS potential at the developer roller and the charge potential at the development point on the photoconductor roller) repels the charged toner particles. The inked image rotates with the photoconductor roller up to a transfer point at which the inked image is transferred onto a transfer roller. The print image can be transfer printed from the transfer roller onto the recording medium.
Corresponding to offset printing, given electrographic printing in digital printing the print images can be constructed from macrocells that respectively comprise microcells or raster cells, wherein raster points or pixels in the raster cells can be generated via exposure of the raster cells on the photoconductor, which raster points or pixels can then be developed by toner. This method has been explicitly explained in U.S. Pat. No. 5,767,888 A, and this is therefore referenced. In what is known as this raster method, the color gradation of the print images from paper color up to the full tone of a primary color can be achieved by adding additional raster points to a raster point of the color of the same thickness. The raster points thus grow step by step within the raster dimensions. The point size of the raster points can thereby be modulated by the character generator via the exposure energy of the photoconductor exposure. The modulation of the exposure energy in a raster point is thus used in order to initially adjust the size of a raster point or pixel. If a raster point has already been exposed with the highest possible exposure energy and an additional inking of the macrocell is required, a raster point or multiple adjacently situated raster points can then be used for raster formation, and their exposure can be modified step by step (thus U.S. Pat. No. 5,767,888 A).
This raster method has the following core points:                The toner application is of nearly the same thickness both in raster points and in solid areas.        The color gradation of print images is achieved via a raster made up of raster points that are more or less fine (and accordingly visible).        Shaded elements of print images are rastered; their edges are accordingly rough and inexact, in particular given an angling of these elements.        